A variety of insulating heat-resistant materials suitable for use in electrical and mechanical insulation are well known in the prior art. Of the insulating heat-resistant materials utilized in the process of casting metals that are known in the prior art, portland cement based calcium silicate materials have proven to be of particular utility.
A fundamental formulation and process for making calcium silicate insulating material is disclosed in U.S. Pat. Nos. 4,111,712 and 4,128,434 to Pusch. This fundamental calcium silicate insulating material is produced by combining, a source of calcium, such as hydrated lime or quick lime, a source of siliceous material, such as silica, diatomaceous earth, silica fume, colloidal silica, or other suitable oxides of silicon, fibrous wollastonite and an organic fiber, such as kraft made from wood pulp, in the presence of at least one part by weight water per part by weight of the forgoing combined materials, to form an aqueous slurry. The aqueous slurry is then poured into a mold where the excess water is pressed out of the mixture to form an uncured shape, typically a sheet. The uncured shape is then placed in an autoclave where it is heated under steam pressure of about 100 psi. Finally, the resultant calcium silicate insulating material is cut or machined to the appropriate dimensions for use in the particular application.
In producing a high density calcium silicate insulating material, portland cement has herebefore been utilized as the calcium source in the formulation. In high density calcium silicate insulating materials, it is desirable that the insulating material have high density, increased strength, good machining characteristics, and be homogeneous throughout with minimized thermal shrinkage. Of particular importance for calcium silicate insulating material utilized in connection with insulating electrical equipment is structural durability. In the past, asbestos fibers had been utilized as a reinforcing fiber in manufacture of calcium silicate insulating materials to provide sufficient strength and toughness to the insulating material. Although such asbestos containing insulating materials performed well, the use of asbestos fibers has been widely discontinued due to health and environmental concerns.
U.S. Pat. No. 5,073,199 to Krowl et al. discloses a calcium silicate insulating material containing pitch based graphite fiber to provide toughness and strength to the insulating material. However, the incorporation of such graphite fiber and its associated material cost results in an appreciable increase in the cost of the resultant product.
Accordingly, it is the principle objective of the present invention to provide an electrical insulating material that is suitable for use in electrical insulating applications with greater refractoriness, is tough and resistant to high temperature cracking, and does not possess the shortcomings of the prior art insulating materials.
An additional objective of the present invention is to provide a tough asbestos-free fire resistant, heat insulating, electrical insulating, and corrosion resistant material, that may be utilized in other applications, such as metal casting, having reduced health exposure risk and minimal environmental impact.
Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description of the invention.